The colon and other vertebrate epithelia serve to regulate and maintain the composition of body fluid compartments by virtue of their ability to effect the active transport of solutes and to act as higly selective barriers to the exchange of solutes and water. The functional characteristics of such epithelial tissues appear to arise from the asymmetric properties of the apical and basolateral membranes of a single layer of cells in conjunction with a paracellular shunt path. The object of the proposed studies is to examine the properties of a simple epithelium, the turtle colon, which may serve as a useful model for the function of the mammalian colon and renal distal tubule. We will attempt to obtain information as to the physical properties of the apical, basolateral and junctional membranes of this epithelium by determining directly their ion and nonelectrolyte selectivity under a variety of conditions. The properties of the apical and basolateral membranes will be determined by the measurement of the initial rates of isotope uptake from the mucosal or serosal bathing solution into the cells, by direct measurement of intracellular electrical potentials, by determination of steady-state intracellular composition, and by measurement of trans-epithelial isotope flows, ionic current and conductances. We will examine the effects of specific chemical agents which modify the properties of biological membranes and attempt to determine what physical properties of epithelial membranes govern their ionic selectivity. We will attempt to test the hypothesis that a primary action of aldosterone is an alteration in the physical properties of the apical membrane of Na-transporting epithelia, and we will attempt to measure directly the effects of vasopressin on the properties of the cation channels and the lipid portion of apical cell membranes.